Brothers and sisters you have none but that mans father is your fathers son?

Answer 1

The person in question is your son.

Think about it this way:

Who is "your father's son"? It's you, because you have no brothers or sisters. So, you just replace "your father's son" with "you", and the riddle now reads: "...that man's father is you." Simple.

Answer 2

Riddle: A man is holding a photograph of a man and says "brothers and sisters I

have none but this mans father is my fathers son". Who is the man in the photo?

Answer: The man in the photo is his son.

This man's father, the man in the photograph's father, "is my father's son". The man in the photograph is the son of someone who "is my father's son". The man holding the photograph has no brother's or sisters. His father's son is himself. He is the father of the man in the photograph. The man in the photograph is his son.